1. Technical Field of the Invention
The present invention relates in general to circuits, and more particularly to a demultiplexing and clock recovery circuit for high data rate signals.
2. Background of the Invention
Optical time division multiplexing (xe2x80x9cOTDMxe2x80x9d) is a desirable technique for increasing the fiber transmission capacity beyond the limits set by the bandwidth of transmitter and receiver electronics. In this technique, a high line rate OTDM signal is generally realized by bit interleaving a number, n, of tributary signals of lower data rate, ft.
As generally known to those skilled in the art, optical demultiplexers and clock recovery circuits are important elements in OTDM systems for retrieving the tributary rate (xe2x80x9cftxe2x80x9d), from the line rate (xe2x80x9cn*ftxe2x80x9d), as well as the tributary clock frequency at ft.
The prior art OTDM techniques generally use separate elements to perform demultiplexing and clock recovery. For example, electro-absorption (xe2x80x9cEAxe2x80x9d) modulators are predominantly used for demultiplexing because they have proven to be very efficient as optical demultiplexers of high data rate OTMD signals, whereas clock recovery has been based on the nonlinear response of semiconductor optical amplifiers in combination with electronic phase locked loops. Alternatively, injection locking of lasers have been utilized for clock recovery.
Some of the disadvantages associated with using separate elements or devices for demultiplexing and clock recovery may be an increase in the amount of components that may result in a more costly and complicated system such that systems may further have an increased rate of failure.
Therefore, it is highly desirable to have a single device that can both demultiplex data signals as well as recover clock frequency of those data signals.
To overcome the shortcomings of the existing prior art devices, the present invention is directed to an optical element that can retrieve the tributary data rate and the clock frequency from the line rate of an OTDM signal. In the present invention, the demultiplexing and clock recovery principle is based on injection locking of a high-Q-filtered and high gain loop with a variable phase delay and an EA-modulator with high non-linear response, i.e., absorption verses applied voltage.
In accordance with the goals of the present invention, there is provided a modulator that is preferably an EA-modulator, an optical amplifier preferably an erbium doped fiber amplifier (xe2x80x9cEDFAxe2x80x9d) or a semiconductor optical amplifier (xe2x80x9cSOAxe2x80x9d), a base band receiver, an electronic amplifier, a high-Q filter, and a variable phase delay. The bandwidth of the basic elements need not exceed the tributary signals of lower data rate, ft.
An aspect of the present invention includes a non-linear modulator for receiving a tributary signal, a first optical amplifier coupled to the non-linear modulator for amplifying the multiplexed signal. A baseband receiver is coupled to the first amplifier, the baseband receiver for converting the received optical signal to an electrical signal. An electronic amplifier coupled to the baseband receiver amplifies the received signal, and a high-Q filter coupled to the electronic amplifier retrieves signal of selected frequency range. A variable phase delay coupled to the high-Q filter, and also to the non-linear modulator, adjusts clock signal phases in the selected frequency range to recover clock frequency. The phase corrected signal from the high-Q filter is also fed back to the non-linear modulator.
Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.